1. Field of the Invention
The present invention relates to an electric power steering circuit assembly for the auxiliary energizing of an automotive steering assembly by the torque of a motor, and relates in particular to the construction of the circuit.
2. Description of the Related Art
As shown in FIG. 8, an automobile provided with an electric power steering circuit assembly generally comprises: a torque sensor 50 for detecting steering torque from a steering wheel 30; a vehicle speed sensor 51 for detecting the speed of the vehicle; and an electric power steering circuit assembly 100 for making a motor 40 output auxiliary torque in a required direction in response to steering torque and vehicle speed.
FIG. 9 is a circuit diagram for a generic electric power steering circuit assembly showing part thereof as a block diagram.
In FIG. 9, 40 is the motor for outputting auxiliary torque with respect to the steering wheel of an automobile (not shown), and 41 is a battery for supplying a motor current IM to drive the motor 40.
42 is a high-capacitance (approximately 3600 .mu.F) capacitor for absorbing ripple components in the motor current IM, 43 is a shunt resistor for detecting the motor current IM, 44 is a bridge circuit comprising a plurality of semiconductor switching components Q1 to Q4 (field-effect transistors (FETs), for example) for switching the motor current IM in response to the magnitude and direction of the auxiliary torque, and 49 is a coil for eliminating electromagnetic noise.
L1 is a lead for connecting one end of the capacitor 42 to ground, P1 and P2 are wiring patterns for bridge connection of the semiconductor switching components Q1 to Q4 as well as for connecting the shunt resistor 43 to the bridge circuit 44, and P3 is a wiring pattern constituting an output terminal for the bridge circuit 44.
45 is a connector comprising a plurality of lead terminals for connecting the motor 40 and the battery 41 to the bridge circuit 44, L2 is external wiring for connecting the motor 40 and the battery 41 to the connector 45, 46 is a normally open relay for switching off the motor current IM when necessary, P4 is a wiring pattern for connecting the relay 46, the capacitor 42, and the shunt resistor 43, and P5 is a wiring pattern for connecting the connector 45 to ground. The wiring pattern P3 constituting an output terminal for the bridge circuit 44 is connected to the connector 45.
47 is a drive circuit for activating the motor 40 by means of the bridge circuit 44 as well as for activating the relay 46, L3 is a lead for connecting the drive circuit 47 to the excitation coil of the relay 46, L4 are leads for connecting the drive circuit 47 to the bridge circuit 44, 48 is a motor current detection means for detecting the motor current IM by means of one end of the shunt resistor 43, and the drive circuit 47 and the motor current detection means 48 constitute peripheral circuit components of a microcomputer described below.
50 is a torque sensor for detecting the steering torque T of the steering wheel, and 51 is vehicle speed sensor for detecting the vehicle speed V of the automobile. 55 is a microcomputer (ECU) for calculating the auxiliary torque based on the steering torque T and the vehicle speed V as well as obtaining feeding back concerning the motor current IM to generate a drive signal corresponding to the auxiliary torque, and the microcomputer inputs a rotational direction command DO and a current control variable 10 for controlling the bridge circuit 44 into the drive circuit 47 as a drive signal.
The microcomputer 55 comprises: a motor current determining means 56 for generating a rotational direction command DO for the motor 40 and a motor current command Im corresponding to the auxiliary torque; a subtraction means 57 for calculating the current variance .DELTA.I between the motor current command Im and the motor current IM; a PID calculating means 58 for calculating compensation for a P (proportional) term, an I (integrated) term, and a D (differential) term, from the current variance .DELTA.I to generate the current control variable 10 corresponding to the PWM duty ratio.
Furthermore, although not shown, the microcomputer 55 includes a commonly-known self-diagnostic function in addition to an analog-digital converter (ADC), PWM (Pulse-Width Modulation) timer circuit, etc., and continuously diagnoses itself to make sure that the system is operating normally. If an abnormality arises, the relay 46 is opened by means of the drive circuit 47, switching off the motor current IM. L5 are leads for connecting the microcomputer 55 to the drive circuit 47.
Generally, the circuit elements 42 to 44, and 49, the wiring patterns P1 to P5, and the leads L1 and L2 interposed between the motor 40 and the battery 41 are enlarged in order to cope with the large motor current IM out of consideration of heat dissipation (thermal resistance), durability, etc., as described below. On the other hand, the microcomputer 55, the peripheral circuit components including the drive circuit 47 and the motor current detection circuit 48, and the leads L3 to L5, are reduced in size since high density is required to handle the small currents therein.
FIG. 10 is a plan view showing the circuit layout of a generic electric power steering circuit assembly, Q1 to Q4, 42, 43, 45, 46, 49, and 55 corresponding to parts with the same numbers in FIG. 9. In this case, the semiconductor switching components Q1 to Q4 each comprise a pair of FETs coated with resin, the high-capacitance capacitor 42 comprises three capacitors, and the microcomputer 55 consists of a single integrated circuit (IC) chip. Furthermore, in order to avoid complicating the diagram, peripheral circuit components, wiring patterns, leads, etc., have been omitted and only the representative structural elements are shown.
In FIG. 10, 1 is a box-shaped metal frame functioning as both a shield plate and a heat sink, 2 is an insulated printed circuit board mounted on the floor of the metal frame 1, and 3 are heat sinks of aluminum, for example, one side of each of which is joined to an inner surface of the metal frame 1. The circuit elements 42, 43, 46, 49, 55, etc., are each mounted on the insulated printed circuit board 2, and the semiconductor switching components Q1 to Q4 are each joined to the other side of the heat sink 3.
4a to 4e constitute conductor strips corresponding to the wiring patterns P1 to P5, etc., employing conductor strips wider and thicker than those of the wiring patterns on the insulated printed circuit board 2 specifically in order to cope with the large current.
Next, the operation of the conventional electric power steering circuit assembly shown in FIG. 10 will be explained with reference to FIG. 9.
The microcomputer 55 receives information concerning the steering torque T and the vehicle speed V from the torque sensor 50 and the vehicle speed sensor 51, respectively, and obtains feedback input concerning the motor current IM from the shunt resistor 43 to generate a rotational direction command D0 for power steering and a current control variable I0 corresponding to the auxiliary torque, and inputs these into the drive circuit 47 by means of the leads L5.
Under regular driving conditions, the drive circuit 47 closes the normally open relay 46 by a command sent through the lead L3, and when the rotational direction command D0 and the current control variable I0 are input generates a PWM activation signal and applies it to the semiconductor switching components Q1 to Q4 of the bridge circuit 44 by means of the leads L4.
Thus, the motor 40 is driven by the motor current IM supplied from the battery 41 through the external wiring L2, the connector 45, the coil 49, the relay 46, the wiring pattern P4, the shunt resistor 43, the wiring pattern P1, the bridge circuit 44, the wiring pattern P3, the connector 45, and the external wiring L2, outputting the required amount of auxiliary torque in the required direction.
At that time, the motor current IM is controlled to match the motor current command Im by detecting the motor current IM by means of the shunt resistor 43 and the motor current detection means 48 and by feeding this information back to the subtraction means 57 in the microcomputer 55. Furthermore, the motor current IM contains ripple components due to the switching operation of the bridge circuit 44 during activation of the PWM, but these are smoothed and controlled by the high-capacitance capacitors 42. In addition, the coil 49 prevents noise generated by the switching operation of the bridge circuit 44 during activation of the PWM from being externally emitted and becoming radio noise.
The value of the motor current IM controlled by an electric power steering circuit assembly of this kind is approximately 25 A even in a light automobile, and may reach approximately 60 to 80 A in a small automobile. Consequently, it is necessary to suppress generation of heat by the semiconductor switching components Q1 to Q4 constituting the bridge circuit 44 while switched on and during switching of the PWM by enlarging the components to cope with the magnitude of the motor current IM as well as connecting a plurality thereof in parallel as shown.
Furthermore, the heat sink 3 is required in order to dissipate the heat generated by the semiconductor switching components Q1 to Q4, and the greater the motor current IM, the greater the number of semiconductor switching components Q1 to Q4 required, requiring that the heat sink 3 be enlarged accordingly.
In addition, the lengths of the wiring patterns P1, P2, and P4 from the terminals of the connector 45 to ground via the coil 49, the relay 46, the shunt resistor 43 and the bridge circuit 44, and the length of the wiring pattern P3 from the bridge circuit 44 to the motor 40 physically increase in proportion to increases in the motor current IM, increases in the number of semiconductor switching components Q1 to Q4, and enlargement of the heat sink.
As a result, in a conventional electric power steering circuit assembly, thick, wide conductor strips 4a to 4e specifically designed for large currents are used, as shown in FIG. 10, because there is a risk that the wiring patterns P1 to P4 will lose thermal resistance and durability if temperatures increase dramatically due to heat generated as result of voltage drops in each of the wiring patterns P1 to P4. This consequently leads to enlargement of the insulated printed circuit board 2.
Furthermore, the capacitors 42, the shunt resistor 43, the relay 46, and the coil 49 are enlarged because of increases in the motor current IM, and attempting to mount these on the insulated printed circuit board 2 leads to further enlargement of the insulated printed circuit board 2 to provide mounting space.